A. Field of the Invention
The present invention relates to a process for producing a polyolefin-based resin expanded beads molded article and to a polyolefin-based resin expanded beads molded article. More specifically, the present invention is directed to a process for producing a polyolefin-based resin expanded beads molded article having interconnected void spaces and to a polyolefin-based resin expanded beads molded article.
B. Background Art
An expanded molded article of a polyolefin-based resin having void spaces excels in properties such as water permeability, gas permeability and sound absorbing property and has proper rigidity and elasticity and, therefore, is used as a drainage material, a wall material for buildings, an interior material for automobiles, etc. As the range of uses of such polyolefin-based resin expanded molded articles having interconnected void spaces is widened in recent years, requirements for performance of molded articles with a porosity further increase. Thus, molded articles with a high porosity are demanded in order to improve their characteristics such as water permeability, gas permeability and sound absorbing properties. Moreover, there is a demand for expanded beads molded articles having high fusion bonding strength between expanded beads and excellent bending deflection characteristics from the viewpoint of durability and impact resistance.
As a method for producing a polyolefin-based resin expanded beads molded article having a high porosity, there has been hitherto known a method in which cylindrical thermoplastic resin expanded beads each having a throughhole are molded into an expanded beads molded article.
For example, JP-A-H08-108441 discloses production of a molded article having void spaces using polyolefin-based resin expanded beads each having a throughhole.
JP-A-2004-68016 discloses the use of cylindrical polyolefin-based resin expanded beads each having a core layer formed of a polypropylene-based resin and an outer layer formed of a polypropylene-based resin having lower melting point than that of the core layer for producing an expanded beads molded article having a high porosity.
Molding of the expanded beads disclosed in JP-A-H08-108441 in a mold cavity can give an expanded beads molded article having void spaces. When an expanded beads molded article having a higher porosity and a low bulk density is produced with this method, however, it is necessary to perform the in-mold molding under conditions which can prevent the expanded beads from undergoing secondary expansion in order to avoid a reduction of the porosity. Thus, JP-A-H08-108441 is problematic when an expanded beads molded article having high fusion bonding strength and, therefore, excellent bending deflection characteristics is intended to be produced.
Molding of the expanded beads disclosed in JP-A-2004-68016 in a mold cavity can achieve higher porosity and higher fusion bonding strength than those attained by conventional methods. Even when the expanded beads are molded in the mold cavity after having been subjected to an inside pressure increasing treatment for improving their secondary expansion properties, however, it is still difficult to obtain a light-weight polyolefin-based resin expanded beads molded article having excellent bending deflection characteristics, a high porosity and a lower bulk density.